My invention relates to codecs (coder-decoders), primarily intended for encoding and decoding of analog signals, e.g., speech of similar signals, in Pulse Code Modulation (PCM) systems, and is particularly applicable to so-called per-line codecs which are not time-shared by several PCM channels.
A particular and commonly-used category of such codecs relies on so-called digital-to-analog converters (DAC's) for encoding and decoding. The DAC usually comprises a resistor network in combination with a number of electronic switches which are set by the digital code to be converted. The analog output signal from the combination network will then be related to a reference input signal according to the setting of the switches and the desired transfer characteristic. This reference signal is usually a fixed voltage of selectable polarity.
In order to accommodate great variations in the signal levels to be encoded, while yet maintaining an acceptable signal-to-quantization-noise ratio, compressed pulse code modulation (CPCM) employing companding is customarily used. This may follow one of the companding laws accepted as standard in the communications industry, e.g., the so-called Mu-Law or the so-called A-Law. As is well-known, companding requires a nonlinear transfer characteristic between the input signal and the resulting CPCM code. An inverse transfer characteristic is applied in the decoding process in order to provide an overall transfer charactertistic that is linear after the speech signal has been both coded and decoded. This requirement can be met, at least approximately, by using identical DAC's for both encoding and decoding. Since the DAC is one of the most complex and costly components of a PCM system using per-line codecs, such standardization of the DAC for both the encoding and the decoding processes is highly desirable.
Even more desirable from the standpoint of economy and circuit simplification would be the use of a single DAC for both encoding and decoding in each line. However, upon closer examination of such multiple use of a DAC in a codec, one encounters a dilemma. Regardless of whether companding is used or not, the decision levels during encoding and the reconstruction levels during decoding do not coincide. As a matter of fact, in order to achieve minimum overall signal degradation in an end-to-end connection, the reconstruction levels during decoding should be exactly midway between the corresponding decision levels during encoding. Thus, it has previously been considered necessary to use separate DAC's for encoding and decoding, or else to add complexity to the DAC so that its transfer characteristic can be modified by a control signal between the encode and decode modes.